In a conventional suspension system for a steerable wheel, the knuckle which rotatably supports a wheel is typically pivoted at its upper and lower points, respectively, to the free ends of, for instance, a pair of A-arms which are pivotally connected to the vehicle body at their base ends so as to allow a vertical motion and a steering action of the wheel. In such a suspension system, an imaginary king pin can be defined as a line connecting the upper and the lower pivot points of the knuckle. The wheel is steered about this imaginary king pin but, since the center of the tread contact surface of the wheel and the point of intersection between the imaginary king pin and the tread contact surface generally do not coincide with each other, the so-called self aligning torque which tends to restore the wheel to a straight ahead position is normally produced.
When a manual steering system is used, this self-aligning torque is naturally transmitted to the steering wheel and gives the feel of the state of the steered wheels to the driver. Even when a power assisted steering system is used, the reaction which the wheels experience from the road is allowed to be transmitted from the steered wheel to the steering wheel so as to improve the handling of the vehicle by giving the feel of the steered wheel to the driver. In either case, this self-aligning torque is required to be controlled to a manageable level and should not increase or decrease abruptly as the steering angle is changed.
In the case of a steerable rear wheel of a vehicle equipped with a four-wheel steering system in which the ratio of the steering angle of the rear wheels to the steering angle of the front wheels is varied depending on the speed of the vehicle or the steering angle of the front wheels, the self-aligning torque may change in a highly complex way without offering any benefit and it is desirable to totally eliminate the self-aligning torque if possible.
When the king pin, real or imaginary, is fixed as is the case with most of the conventional suspension systems, the self-aligning torque can be easily eliminated by allowing the king pin intersect with the center of the tread contact surface of the wheel but when that cannot be done because of other considerations then it is impossible to control the self-aligning torque.
On the other hand, in a suspension system of the kind disclosed in Japanese Patent Publication No. 52-9889 having two pairs of lateral links connected to upper and lower parts of the knuckle, respectively, it is possible to change the various parameters of the suspension system and favorable settings of the camber angle, the toe angle and caster angle can be obtained as desired, but since the imaginary king pin moves as the steering angle changes and the resulting change in the self-aligning torque or the force required to maintain a steering angle tends to vary a great deal or in a complicated fashion and may not be sufficiently manageable.